In the recent drive for higher integration and operating speeds in LSI devices, the pattern rule is made drastically finer. The background supporting such a rapid advance is a reduced wavelength of the light source for exposure. The change-over from i-line (365 nm) of a mercury lamp to shorter wavelength KrF excimer laser (248 nm) enabled mass-scale production of dynamic random access memories (DRAM) with an integration degree of 64 MB (processing feature size ≦0.25 μm). To establish the micropatterning technology necessary for the fabrication of DRAM with an integration degree of 256 MB and 1 GB or more, the lithography using ArF excimer laser (193 nm) is under active investigation. The ArF excimer laser lithography, combined with a high NA lens (NA≧0.9), is considered to comply with 65-nm node devices. For the fabrication of next 45-nm node devices, the F2 lithography of 157 nm wavelength became a candidate. However, because of many problems including a cost and a shortage of resist performance, the employment of F2 lithography was postponed. ArF immersion lithography was proposed as a substitute for the F2 lithography. Efforts have been made for the early introduction of ArF immersion lithography (see Proc. SPIE, Vol. 4690, xxix, 2002).
In the ArF immersion lithography, the space between the projection lens and the wafer is filled with water and ArF excimer laser is irradiated through the water. Since water has a refractive index of 1.44 at 193 nm, pattern formation is possible even using a lens with NA of 1.0 or greater. Theoretically, it is possible to increase the NA to 1.44. The resolution is improved by an increment of NA. A combination of a lens having NA of at least 1.2 with ultra-high resolution technology suggests a way to the 45-nm node (see Proc. SPIE, Vol. 5040, p 724, 2003).
Several problems arise when a resist film is exposed in the presence of water. For example, profile changes occur because the acid once generated from a photoacid generator and the basic compound added to the resist can be partially dissolved in water. The pattern collapses due to swelling. It is also pointed out that water droplets remaining on the resist film, though in a minute volume, can penetrate into the resist film to generate defects. To overcome these drawbacks of the ArF immersion lithography, it was proposed to provide a protective coating between the resist film and water using a fluorinated material (see the 2nd Immersion Workshop, Resist and Cover Material Investigation for Immersion Lithography, 2003).
Among fluorinated protective coating materials, protective coatings made of perfluoroalkyl compounds use fluorocarbons as the solvent for controlling a coating thickness and for stripping the protective film after exposure. As is well known, the use of fluorocarbons is a consideration in view of environmental protection. In addition, special units must be added for the coating and stripping of protective film. Fluorocarbons raise serious problems on practical use.
One means proposed for mitigating practical drawbacks of the above-mentioned protective film is a protective coating of the type which is soluble in alkaline developer (JP-A 2005-264131). The alkali-soluble protective film is epoch-making in that it eliminates a need for a special stripping unit because it can be stripped off at the same time as the development of a photoresist film. However, the solvent necessary for coating of protective film material leaves a room for improvement on a practical level because those solvents in which the photoresist layer is dissolved should be avoided, and a special unit for coating of a protective film is needed.
For preventing water from penetrating into the resist film, it is proposed in JP-A 2006-48029 to add an alkali-soluble, hydrophobic compound to the resist material. This method is advantageous over the use of a resist protective film because steps of forming and stripping the protective film are unnecessary. However, when a hydrophobic compound is added to the resist material, the resulting resist film on its surface has an increased contact angle, especially after development, tending to produce defects known as “blob defects.” It is thus desired to have an additive to a resist material which can reduce the contact angle on the resist surface after development while maintaining a high water-barrier property.
When EB lithography is carried out for mask image writing or the like, the resist changes its sensitivity due to evaporation of the acid generated during image writing, evaporation of vinyl ether produced by deprotection of acetal protective groups, or the like, as discussed in JP-A 2002-99090. It is thus desired to have an additive for suppressing such resist sensitivity variation.